The projection lenses of microlithographic projection systems are generally telecentric at least in image space so that the projected images are less sensitive to focusing errors, e.g., maintain the same magnification through the depth of focus. Particularly when partial coherence illumination is provided, design considerations for telecentricity extend into the performance of illuminators of the projection lenses. The effective telecentricity of the projection systems is then determined by the illuminator's angular radiance distribution at the image plane of the projection lens. In telecentric image space, centroids of energy within the angular radiance distributions at the image points extend normal to the image plane. The illuminator image plane, which is located at the object plane of the projection lens, corresponds to a uniform irradiance plane formed by the illuminator. The field of the uniform irradiance is commonly limited within the illuminator by imaging a field stop, such as adjustable blades or the edges of the output face of a kaleidoscope, onto the illuminator image plane.
Whether the projection lenses are singly telecentric (telecentric in image space) or doubly telecentric (telecentric in both image and object space), chief rays illuminating field points on the reticle or mask pattern arrive at conjugate field points on the substrate normal to the surface of the substrate. Departures from telecentricity in the object space of the reticle or mask can be made to support magnification adjustments, such as by axial translations of the mask or reticle within the object space. The illumination pattern at the image plane of the illuminator can be arranged to match the desired departure from telecentricity in the object plane of the projection lens to preserve telecentricity within the image plane of the projection lens.
For some projection lenses, even those intended to be nominally telecentric in object space as well as image space, complex or higher order departures from telecentricity may be required at the output of the illuminator to achieve the desired telecentricity in the image space of the projection lens. The required departures from telecentricity in the image space of the illuminator can add considerable complexity to the design of illuminators and can also disrupt irradiance uniformity, which is also required at the image plane of the projection lens.